Commercial optical systems use optical fibers to carry large amounts of multiplexed digital data over long distances from a transmitting terminal to a receiving terminal. The maximum distance that the data can be transmitted in the fiber without amplification or regeneration is limited by the signal loss and dispersion associated with the optical fiber. To transmit optical signals over long distances, the lightwave systems normally include a number of repeaters periodically located along the fiber route from the transmitting terminal to the receiving terminal. Each repeater boosts the optical input signal to compensate for the transmission losses which occurred since passing through the last previous repeater. Prior to the widespread availability of efficient optical amplifiers, many systems employed repeaters which converted the optical signals into electrical signals for amplification by conventional electrical amplifiers. The amplified electrical signals were then reconverted to the optical domain, for further transmission along the optical communication path. The advent of reliable and low cost optical amplifiers has obviated the need to convert signals into the electrical domain for amplification.
Optical amplifiers, such as rare earth doped optical fiber amplifiers, require a source of pump energy. In a rare earth doped optical fiber amplifier, for example, a dedicated pump laser is coupled to the doped fiber for exciting the active medium (rare earth element) within the amplifier. At the same time, a communication signal is passed through the doped fiber. The doped fiber exhibits gain at the wavelength of the communication signal, providing the desired amplification. If the optical fiber is doped with erbium, for example, pump energy may be provided at a wavelength of 1480 nm or 980 nm, which coincide with the absorption peaks of erbium.
Optical communications systems often employ a line monitoring system (LMS) to monitor the performance of the repeaters. The line monitoring system includes line monitoring equipment (LME) located in the terminal stations and high-loss loop-back paths (HLLB) in the repeaters and terminals. The HLLBs optically couple the two fibers of a fiber pair (one in each direction of transmission) such that a very small portion of the optical signal originating at a transmitting terminal and being transmitted on one of the fibers of the pair is looped back and coupled into the fiber that is transmitting in the reverse direction back toward the sending terminal. The fundamental quantity measured by the LME is the round-trip loop gain between the LME and each terminal and repeater HLLB on a fiber pair. Through routine analysis of the measured loop gains, the LMS can be used to detect changes in the performance of the portion of the system spanned by the monitored repeaters and terminals over time. In particular, the analysis may reveal that these changes may be due to different causes, such as degradations in pump power, variations in the loss in the amplifier output stage, changes in the fiber loss in the transmission span, and amplifier gain changes, for example.
To recognize line faults and other problems from the analysis of loop gain measurements, as described herein, the transmission system must produce a loop gain behavior under fault and problem conditions which is significantly different from its normal behavior. This is clearly the case under the extreme situations of fiber and/or cable breaks, independent of the repeater design, primarily because loop gain measurements beyond the break show that the system is open (i.e., infinite loss). For other more subtle problems, the capability of locating and identifying the problem depends strongly on the type of optical amplifiers used in the system. Many modem repeater designs employ optical amplifiers which dynamically change their gain to correct for moderate loss changes in the fiber between the repeaters. With such amplifiers, if a loss change occurs in the fiber between two repeaters, the loss change is compensated by the aggregate gain changes that occur in the next several repeaters, each one compensating for successively smaller portions of the fiber loss change, until the entire loss change has been equalized. The larger the loss change to be compensated, the more repeaters it takes to equalize the change. The loop gain measurements through the repeaters that have adjusted their gains will be different from the loop gain measurements through the same repeaters in the nominal case, and it is this difference which can be used to locate the loss change and determine its cause. Note that in a system where moderate loss changes are completely compensated by the automatic gain change in a single repeater, the measured loop gain through that repeater in the increased/decreased fiber loss case is identical to the measured loop gain through that repeater in the nominal case. For such situations, the fact that a change in the fiber loss has occurred is not detectable by comparison of the measured loop gain data for the two conditions.
The LMS is used to establish a baseline level of behavior that characterizes the loop gains in each fiber pair in the communications system in its normal operating state. By periodically monitoring round-trip loop gain changes that occur over time, deviations from the baseline behavior can be measured. Deviations from the baseline behavior are referred to as the signature of the measurement and are often indicative of a problem or fault in the system. Extreme faults include fiber and cable breaks and other problems that result in immediate loss of service. Other problems which are also detectable include amplifier power degradations and other loss variations over time, which may not have an immediate effect on the quality of service.
The HLLB signature is typically determined from the data shown in FIG. 1(a). In FIG. 1(a), the data points represent the values of the measured loop gains for each of the amplifier pairs in the sequence in which the amplifier pairs are encountered along the transmission path. That is, the first data point represents the loop gain from the LME to the first amplifier pair and the second data point represents the gain from the LME to the second amplifier pair, and so on. Curve 10 represents the baseline behavior and curve 12 represents data obtained during routine measurement. The difference between curves 10 and 12, shown in FIG. 1(b), is a visual representation of the signature of the gain measurement.
An ideal signature is a straight horizontal line running through a gain change of 0 dB, indicating that all the loop gain measurements from the amplifier pairs agree exactly with the pre-established baseline. Any deviation from such a signature is indicative of abnormal system operation, which is caused by a primary (critical) fault that stops transmission, such as a cable break, and possibly secondary (non-critical) faults that only degrade system performance, such as a decrease in amplifier gain. The particular nature of the fault in the communications system can often be determined from its signature. For example, a failure of one of the pump lasers driving an optical amplifier pair is characterized by a gain increase followed by a gain decrease that spans approximately six amplifier pairs.
Loop gain signatures are conventionally identified by visual inspection. That is, the nature of a fault is determined by visually comparing the measured signatures against a series of predetermined "library" signatures for which faults have been identified. The "library" signature that best matches the measured signature presumably defines the fault.
It would be advantageous to automate the process of identifying system faults from their loop gain signatures to assist the system operator in locating line faults and degradations which may lead to preemptive repair activity.